Objective: To examine the psychometric properties of a German adaptation of the Weiss Functional Impairment Rating Scale–Parent Report (WFIRS-P) in a clinical sample of children (4-12 years) with externalizing behavior disorders. Method: Data were collected within two clinical trials (N = 264). Factorial validity, reliability, and divergent validity from symptoms of ADHD and oppositional defiant disorder (ODD) were assessed. Results: Confirmatory factor analyses revealed that a bifactor model consistent with the theoretical assumption of a general construct of impairment (total scale) and additional specific factors (subscales) provided satisfactory data fit. Model-based reliability estimates showed that both the general construct and specific factors accounted for item variance. Internal consistencies were >.70, part–whole corrected item–scale correlations mostly >.30. Correlations between the WFIRS-P Scales and ADHD and ODD symptoms were low to moderate. Conclusion: The results support the factorial validity, reliability, and divergent validity of the WFIRS-P.